The present invention relates to a manhole closure assembly, and more particularly to a closure that prevents drainage and storm water from emptying into a manhole that leads to a sewerage system.
It is desirable to segregate sanitary or sewerage systems from those systems designed to handle storm and drainage water since such segregation reduces an unnecessary burden on the sewerage treatment facility. As is well known, when drainage and storm water empties into a sanitary system, the sewerage treatment plant must necessarily handle and treat the storm water in addition to the raw sewage. Standard frames and covers for manholes that lead to sanitary or sewerage systems generally include heavy cover constructions with passageways therein that serve to vent the overall system for the purpose of relieving internal pressure as well as vacuum buildups. The passageways in these manhole covers also enable storm water to infiltrate the sewerage system. It is impossible to simply seal off or otherwise eliminate the cover passageways since pressure developed within the system could easily produce a force sufficient to blow the cover away from its seated position at the manhole opening or blast the plugs away from the passageways. Additionally, pressure developed within the sewerage system could easily cause damage to the system if that pressure is not relieved.
With an oversized sewerage or sanitary system, storm water infiltrating the system does not prevent any major problems. On the other hand, with many existing systems capacity is already at peak level, and it is estimated that 40% of that capacity is the direct result of storm or drainage water. For the most part, storm water volume draining into sewerage systems is much more acute today than it was years back, and the key factor is that today there is significantly less pervious area than there was even several years ago. This factor is due to larger impervious street areas, larger parking lots, increased construction and smaller building lots, all of which cut down on the amount of overall pervious area heretofore available to absorb storm and drainage water. These factors have significantly attributed to the problem of overtaxing present sewerage systems. By significantly preventing storm and drainage water from entering into such systems, existing facilities can easily handle an additional 60% or more sewerage capacity.
Manhole closure assemblies are described in U.S. Pat. No. 3,798,848, granted Mar. 26, 1974, and U.S. Pat. No. 3,969,847, granted July 20, 1976. These closure assemblies require a separate support ring normally located directly below the cover supporting flange of an annular manhole frame. However, when the heavy manhole cover is manipulated for removal, it often spins or rolls into the manhole where the cover strikes the closure assembly causing damage and/or knocking it into the manhole. Also, installation of the assemblies described in these patents is a time consuming procedure.
U.S. Pat. No. 3,712,009, granted Jan. 23, 1973, also describes a manhole closure assembly comprising a plurality of parts. Installation of this assembly is time consuming and there is a real danger of parts falling into the manhole during the installing procedure. Since the closure assemblies of the above noted patents also include a plurality of parts, there is a real danager of the parts falling into the manhole during installation of these assemblies too.